1. Field of the Invention
The present invention relates to self-reducing, cold-bonded pellets used in the ferrous industry and their process of production. Because of the self-reducing properties of the pellets when the cold-bonded pellets are heated to high temperatures, iron and steel can be more efficiently produced using the pellets and the pellets can be used in most smelting furnaces such as electric arc furnaces (“EAFs”), converting furnaces, open-hearth furnaces for steelmaking, blast furnaces, non-blast furnaces for iron-making, and direct reduction iron (“DRI”) furnaces for producing DRI products.
2. Description of the Related Art
U.S. Pat. No. 3,150,958 to Collin et al discloses a process for the reduction of iron oxide and powdered carbonaceous material and the general principles for use of self-reducing pellets in steelmaking.
U.S. Pat. No. 3,174,846 to Brisse et al discloses a method of briquetting iron oxide fines with bituminous coal and discloses their use as blast furnace charge material.
U.S. Pat. No. 3,264,092 to Ban discloses a system for producing carbonized and metallized iron ore pellets suitable for use in a smelting operation such as a cupola type furnace or a blast furnace.
U.S. Pat. No. 3,323,901 to Dahl et al discloses pellets made of finely divided metal oxide ore, a carbonaceous reducing agent and a sulphite lye or molasses binder.
U.S. Pat. Nos. 3,437,474 and 3,617,254 to Imperato disclose a method of making lump ore from metal ore/alkaline earth metal oxides and hydroxides/carbonaceous material (e.g., coal) lumps reacted with carbon dioxide in the presence of moisture which are suitable for use in steelmaking furnaces.
U.S. Pat. No. 3,490,895 to Svensson discloses a process for the cold-hardening of pellets containing iron ore concentrates, finely divided Portland cement clinkers and water. The green pellets are embedded in a mass of discrete finely divided iron ore particles. The embedding mass is separated from the pellets when they have achieved an acceptable strength.
U.S. Pat. No. 3,938,987 to Ban discloses pellets formed from iron oxide ore with a deficiency of non-agglomerating and wherein these pellets are sintered on a traveling grate machine in the presence of externally supplied carbonaceous material in the sinter bed sufficient to make up the deficiency within the pellets.
U.S. Pat. No. 4,049,435 to Lotosh et al discloses mixing an ore with a mineral hydraulic binder, the mixture obtained is simultaneously homogenized and activated, then the mixture is pelletized and the green lumps are subjected to a heat humidity treatment followed by a two-stage heat treatment.
U.S. Pat. No. 4,093,448 to Eliseev et al discloses preparing a mixture from ore concentrates with a moisture content of 7 to 15% and with particle size less than 0.83 mm and binding material in the form of calcium oxide and magnesium oxide. The mixture is then hydrated and introduced into ore concentrates to produce a homogeneous mixture containing 4 to 15 weight percent of binding material. The homogeneous mixture is then pelletized to produce pellets by curing them in saturated steam.
U.S. Pat. No. 4,168,966 to Furui et al discloses agglomerates for use in a blast furnace containing a cementitious material and formulated to maintain a CaO to SiO2 ratio in a range of from 1.2 to 1.9 and a slag forming ratio in a range of from 13 to 19%. The as formed discrete moist agglomerates are cured without the necessity of a powder matrix prior to introduction to the furnace.
U.S. Pat. No. 4,528,029 to Goksel discloses self-reducing agglomerates of an iron oxide-containing material produced by preparing a moistened mixture of the ore concentrate, a finely-divided natural pyrolyzed carbonaceous material, about 1 to about 30 weight % of a bonding agent, such as burned or hydrated lime, and 0 up to about 3 weight % of a siliceous material, forming green agglomerates from this mixture; and hydrothermally hardening the green agglomerates by contacting them with steam under pressure.
U.S. Pat. No. 4,636,342 to Miyashita et al discloses continuously supplying green pellets containing a carbonating binder into a vertical type reactor to continuously pass the green pellets sequentially through a pre-drying zone, a carbonating zone and a drying zone in the vertical type reactor; blowing a pre-drying gas into the pre-drying zone to pre-dry the green pellets therein; blowing a carbonating gas comprising carbon dioxide gas of from 5 to 95 vol. % and saturated steam of from 5 to 95 vol. % into the carbonating zone to carbonate the carbonating binder contained in the green pellets therein; and blowing a drying gas into the drying zone to harden the green pellets therein.
U.S. Pat. No. 4,846,884 to Shigematsu et al mixes Portland cement, blast furnace cement or blast furnace slag by mixing the binder with iron ore fines to form large blocks. The blocks are then cured or hardened, and crushed. The disadvantage of this process is that the high temperature compression strength will be lower than what is required, and, also, it is difficult to obtain the self-reduction qualities necessary for the pellets.
U.S. Pat. No. 5,066,327 to Yanaka et al uses cement as a binder and mixes the cement with iron ore fines and/or carbonaceous matter by adding water to form the green pellets. After placing the green pellets on a traveling grate, the pellets are then treated by a gas with a concentration of 55 vol % carbon dioxide or more. The disadvantage of this method is that the concentration of the carbon dioxide needed is too high and rather difficult to find or obtain in such large quantities in an industrial environment.
U.S. Pat. No. 6,334,883 to Takenaka et al discloses pellets containing a carbonaceous material and iron ore mainly composed of iron oxide. The maximum fluidity of the carbonaceous material in softening and melting, and the ratio of iron oxide particles of 10 mu.m or smaller in the iron ore, are within a specified range.
U.S. Pat. No. 6,409,964 to Aota et al discloses shaped bodies containing particulate iron materials, such as cast pellets, briquettes and the like, with sufficient strength to withstand temperatures of up to at least 1000° C. can be obtained by using a fully hydrated high-alumina cement as the binder. The larger-sized particles of the iron ore are used in these pellets, and, therefore, the reduction speed of the pellets is relatively slow. Because of the materials used and production methods, the pellets produced by this process have difficulty in the area of self-reduction. Also, the high alumina content of the binder is not desirable in some melting processes because it will increase the alumina content of the slag.
U.S. Pat. No. 6,565,623 to Contrucci et al discloses curing and drying self-reducing agglomerates containing cement as a binder in the presence of saturated vapor at a temperature from about 70 to about 110° C. and under atmospheric pressure. The self-reducing agglomerates are comprised of mixtures of fines of iron ore and/or industrial residue containing iron oxides and/or metallic iron, fines of carbonaceous materials such as mineral coal, charcoal, green petroleum coke and similar fines, fluxing material such as steel plant slag and blast furnace slag, limestone, lime and similar materials, cement as a binder and fluxing agent, and humidity between 7 and 12%. This process employs steam to cure the cement blocks but because the green blocks have low compression strength, the green blocks must be pre-dried to reduce the water content and thereby attempt to increase the compression strength of the green blocks. However, this method of pre-drying the green blocks will render the green blocks insufficiently hydrated and decrease the quality of compression strength of the final product. The cold compression strength is considered lower than the desired average and only ranges from about 17-50 kgf/pellet.
For the iron and steel smelting techniques being developed at present, such as the direct steel making technique, the smelt-reduction iron-making technique, the DRI technique, the technique of reducing coke-to-metal ratio in blast furnaces and the iron-making technique of using cold-bonded pellets as blast furnace charge instead of sinter, the largest problem encountered is how to produce stable, highly effective and quick reducing iron ore at relatively low cost under all sorts of smelting conditions during industrial production. For this reason, development of cold-bonded agglomerates with self-reducing capability is considered to be an important approach for solving this problem.
There is a process known as the AISI process. The AISI process includes a pre-reduction stage and a smelt reduction stage. In the AISI process, pre-heated and partially pre-reduced iron ore pellets, coal or coke breeze and fluxes are top charged into a pressurized smelt reactor which contains a molten bath of iron and slag. The coal devolatilizes in the slag layer and the iron ore pellets dissolve in the slag and then are reduced by carbon (char) in the slag. The process conditions result in slag foaming. Carbon monoxide and hydrogen generated in the process are post combusted in or just above the slag layer to provide the energy required for the endothermic reduction reactions. Oxygen is top blown through a central, water cooled lance and nitrogen is injected through tuyeres at the bottom of the reactor to ensure sufficient stirring to facilitate heat transfer of the post combustion energy to the bath. The process off gas is dedusted in a hot cyclone before being fed to a shaft type furnace for pre-heating and pre-reduction of the pellets to FeO or wustite.
There is also a process known as the COREX.R™ process (COREX.R™ is a trademark of Deutsche Voest-Alpine Industrieanlagenbau GMBH and Voest-Alpine Industrieanlagenbau). In the COREX.R™ process the metallurgical work is carried out in two process reactors: the reduction furnace and the melter gasifier. Using non-coking coals and iron bearing materials such as lump ore, pellets or sinter, hot metal is produced with blast furnace quality. Passing through a pressure lock system, coal enters the dome of the melter gasifier where destructive distillation of the coal takes place at temperatures in the range of 1,100-1,150° C. Oxygen blown into the melter gasifier produces a coke bed from the introduced coal and results in a reduction gas consisting of 95% CO+H2 and approximately 2% CO2. This gas exits the melter gasifier and is dedusted and cooled to the desired reduction temperature between 800° C. and 850° C. The gas is then used to reduce lump ores, pellets or sinter in the reduction furnace to sponge iron having an average degree of metallization above 90%. The sponge iron is extracted from the reduction furnace using a specially designed screw conveyor and drops into the melter gasifier where it melts to the hot metal. As in the blast furnace, limestone adjusts the basicity of the slag to ensure sulfur removal from the hot metal. Depending on the iron ores used, SiO2 may also be charged into the system to adjust the chemical composition and viscosity of the slag. Tapping procedure and temperature as well as the hot metal composition are otherwise exactly the same as in a blast furnace. The top gas of the reduction furnace has a net calorific value of about 7,000 KJ/Nm3 and can be used for a wide variety of purposes.
The cold bonding process is defined as both a physical and chemical process to produce agglomerates of a predetermined size and with sufficient strength and durability for use. This is accomplished by means of mixing the iron-oxide containing materials, binders, and/or additives to form green pellets by using a pelletizing machine. After the pellets have been pelletized, the green pellets are then usually cured.
The cold bonding process is usually classified by the types of binders used in the pellets or the methods of curing the pellets. For example, the hydraulic bond, the carbonate bond, the thermo-hydraulic bond, the Sorel cement bond, the liquid glass bond and other organic binder bonds are processes that have all been analyzed and utilized in the past, but with less than satisfactory results. Below are two examples of the hydraulic and carbonate cold bonding methods.
The hydraulic bond process uses hydraulic substances as binders. For example, the cement used in this process is comprised of Portland cement, high alumina cement, blast furnace cement, or blast furnace slag. Also used are lime, hydro-lime, and others. After mixing the binders and iron-oxide containing materials and adding water, the pellets are formed. Thereafter, the pellets are dried and hardened. The typical hydraulic bond process uses Portland cement as a binder.
The carbonate bond process uses lime, hydrated lime, or other lime-containing materials as a binder. After mixing the binder and the iron ore fines together to form the pellets, the pellets are then cured by hot gases containing carbon dioxide. The calcium hydroxide contained in the pellets reacts with carbon dioxide to form calcium carbonate and, after this has occurred, the pellets will then gain adequate strength and durability.
All cold bonding methods to date have proved to be ineffectual in practical application. The reason for this is that all these methods possess at least one flaw resulting in the following deficiencies: low cold compression strength, unsatisfactory reduction degradation index, low high temperature strength, inadequate reducibility, high production costs, or failure to produce continuous large quantities of iron or steel for industrial utilization.
Because other known cold-bonded agglomerates cannot meet the exacting requirements of comprehensive metallurgy performance, or realize continuous industrial production at low cost, these existing techniques still cannot be widely applied to the iron and steel industries. The MTU carbonaceous pellets can be partly put into a blast furnace for iron making, and also in an electric furnace for steelmaking as well as a cupola for iron making, but the pellets cannot be put into continuous and large-scale production at low cost and therefore cannot be applied industrially.
Accordingly, it is an object of the present invention to solve the above-mentioned problems with pellets and a process for their preparation which can be put into practical operation.